This invention is related, generally, to the field of fiber optic lasers and amplifiers and, more specifically, to monitoring the output power of such devices.
Fiber optic lasers are known in the art, and are used as coherent optical sources for a number of applications. Similarly, fiber optic amplifiers are known, and are used for boosting the power of a desired optical signal. Both of these devices may use a similar gain mechanism. An optical fiber, often doped with an active material such as a rare earth element, is xe2x80x9cpumpedxe2x80x9d by coupling optical energy into it at a predetermined wavelength. The pumping wavelength is selected according to the characteristics of the fiber, such as an absorption band of a particular dopant that has been used. Pumping energy coupled into the fiber is absorbed, causing a population inversion in the doped material that is followed by optical radiation at a predetermined wavelength. Amplifiers typical rely on the stimulated emission of radiation at the wavelength of a signal input to the fiber. For a fiber laser, the fiber core is provided with reflective ends, such that a resonance condition develops within the fiber cavity. An output coupler, being only partially reflective, allows the output of the developed laser energy. Typically, a wavelength-stabilizing element is used to enhance reflectivity at a desired output wavelength, and thereby select the output wavelength of the laser.
Many fiber optic lasers and amplifiers use a fiber with a single cladding layer, and couple the pumping energy of the laser directly into the fiber core. Alternatively, the fiber may be a double-clad fiber in which the core is surrounded by two different cladding layers, an inner or xe2x80x9cpumpxe2x80x9d cladding surrounding the core, and an outer cladding surrounding the pump cladding. Fibers of this configuration may be pumped by injecting the optical pump energy into the pump cladding layer of the fiber. The pump energy repeatedly encounters the core of the fiber as it undergoes internal reflection within the pump cladding layer, and is absorbed by the dopant of the core, providing the desired population inversion. The use of a double-clad fiber typically allows the coupling of a greater amount of pump energy into the gain medium than is possible with a single-cladding fiber. As such, double-clad fiber lasers are desirable for higher power applications.
To maintain a consistent operation of a fiber laser or amplifier, it is desirable to monitor the output power. This allows fluctuations in the output power to be compensated for by adjusting the pumping power level. In this way, an effective feedback arrangement may be constructed to help maintain a stable output power. To accomplish this, present systems use a low reflectivity beam splitter/photodiode combination located in the termination optics, that is, at the end of the gain medium. These power xe2x80x9ctapsxe2x80x9d allow a certain fraction of the output light to be monitored, and thereby provide an indication of the total output power. However, the use of these devices is costly both monetarily and in the loss of system power. Moreover, they tend to be susceptible to environmental conditions and the particular handling by a system user.
In accordance with the present invention, an optical gain system includes a photodetector that monitors side light emitted through the cladding of the fiber. The detected side light is indicative of the output power of the gain system, and may be used to monitor and control that output power. The gain system is preferably a fiber optic laser or a fiber optic amplifier, and includes an optical fiber and a pumping source that generates optical pumping energy and couples it into the fiber. The detected side light may include optical energy at the signal wavelength, at the pump wavelength or both, and the desired wavelengths may be selected with a filter located between the fiber and the detector. The fiber may have a single cladding, but is preferably double clad, having an inner cladding, into which the pumping energy is. coupled, and an outer cladding. The fiber also has a gain characteristic at a signal wavelength, that is, it generates optical signal energy at the signal wavelength in response to the pump energy. In particular, the fiber core may be doped with at least one active element, such as ytterbium. Alternatively, the fiber core may contain no active element, but provide gain by Raman amplification.
In a preferred embodiment, the fiber is wound on a spool that has an opening adjacent to the fiber. The photodetector is positioned at the end of the opening opposite the fiber so as to receive side light that exits the fiber through the cladding and passes through the spool opening. The photodetector generates a control signal indicative of the side light power level, and outputs the signal to a controller. The controller controls the pumping source, and responds to changes in the control signal by adjusting the power output from the pumping source to the fiber such that the detected side light is maintained at a substantially constant power level. For example, if the power level of the detected side light increases, the controller adjusts the pumping source so as to reduce the amount of pumping energy coupled into the fiber. Similarly, if the power level of the detected side light decreases, the controller adjusts the pumping source so as to increase the amount of pumping energy coupled into the fiber.
The side light used to control the system is preferably at the signal wavelength, although light at the pump wavelength may also be used instead of, or in addition to, the signal light. A filter or filters may be used to control the light reaching the photodetector, and may also serve to remove other, unwanted wavelengths.